Optical data storage media often take form in a data-carrying layer formed on a disk shaped polycarbonate substrate. These devices typically fall in the category of substrate-incident media since data recorded in the data-carrying layer is retrieved by illuminating the data-carrying layer through the substrate. More particularly, illumination light for reading the data first passes through the substrate before being received by the data-carrying layer surface.
Optical data storage media or disks have different properties and can be characterized by their attributes, such as read-only optical data storage media or write-once optical data storage media. In read-only optical data storage media, data is recorded in the data-carrying layer as a series of physical marks or bumps. These physical marks or bumps are typically formed using an injection-molding process. Once formed the physical marks or bumps cannot be erased or overwritten. The physical marks or bumps have different diffractive properties when compared to areas of the read-only data-carrying layer that lack physical marks or bumps. Data retrieval in read-only optical data storage media is facilitated by the difference in diffractive properties.
Write-once optical data storage media commonly take the form of a polycarbonate substrate spin coated with a layer of organic dye. Data is recorded by illuminating the organic dye layer through the substrate with a light beam whose intensity is modulated in accordance with data to be recorded. Select areas of the organic dye subjected to high intensity light chemically change and result in “dark” areas, i.e., areas which have a lower light reflectivity when compared to areas of the organic dye which are not subjected to high intensity light. To achieve compatibility with read-only optical data storage media, data is recorded as a sequence of low reflectivity dark regions each one of which is positioned between spaces of high reflectivity. This difference in reflectivity facilitates data retrieval. The chemical change in the organic dye is irreversible. Thus, data written to the organic dye layer cannot be overwritten.
Utility of the disk can be increased by including both read-only and write-once portions on one or both sides of the disk. However, in order to manufacture such a disk, the organic spin-coated dye is patterned over the disk with separate areas for the read-only portions and write-once portions. This increases the complexity, and thereby the cost, in making the disk. The problem in partially coating a surface is that of shielding over the active surface. The effective shielding of the surface requires that an inner or outer annulus be covered without causing a shadowing effect (e.g., in sputter coated disks) or without dye streaking during spin coating, as in the case of dye based disks. Further, this must be accomplished with a variable degree of coverage from disk content to disk content. In practice, this is very difficult to achieve without affecting process yield and associated scrap cost.
Accordingly, an optical disk is desired that overcomes the disadvantages discussed above with conventional optical disks.